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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/api/basic_json_document/set/" rel="canonical"><link href=../save/ rel=prev><link href=../shrink_to_fit/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>set - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> set </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" 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12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg>Line truncated
|
||||
<span class=k>template</span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>V</span><span class=o>></span>
|
||||
<span class=n>view_type</span><span class=w> </span><span class=n>set</span><span class=p>(</span><span class=n>view_type</span><span class=w> </span><span class=n>target</span><span class=p>,</span><span class=w> </span><span class=n>V</span><span class=o>&&</span><span class=w> </span><span class=n>value</span><span class=p>);</span>
|
||||
|
||||
<span class=c1>// (2)</span>
|
||||
<span class=k>template</span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>V</span><span class=o>></span>
|
||||
<span class=n>view_type</span><span class=w> </span><span class=n>set</span><span class=p>(</span><span class=n>view_type</span><span class=w> </span><span class=n>object</span><span class=p>,</span><span class=w> </span><span class=n>string_view_t</span><span class=w> </span><span class=n>key</span><span class=p>,</span><span class=w> </span><span class=n>V</span><span class=o>&&</span><span class=w> </span><span class=n>value</span><span class=p>);</span>
|
||||
|
||||
<span class=c1>// (3)</span>
|
||||
<span class=k>template</span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>I</span><span class=p>,</span><span class=w> </span><span class=k>typename</span><span class=w> </span><span class=nc>V</span><span class=o>></span>
|
||||
<span class=n>view_type</span><span class=w> </span><span class=n>set</span><span class=p>(</span><span class=n>view_type</span><span class=w> </span><span class=n>array</span><span class=p>,</span><span class=w> </span><span class=n>I</span><span class=w> </span><span class=n>idx</span><span class=p>,</span><span class=w> </span><span class=n>V</span><span class=o>&&</span><span class=w> </span><span class=n>value</span><span class=p>);</span>
|
||||
|
||||
<span class=c1>// (4)</span>
|
||||
<span class=k>template</span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>V</span><span class=o>></span>
|
||||
<span class=n>view_type</span><span class=w> </span><span class=n>set</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json_pointer</span><span class=o>&</span><span class=w> </span><span class=n>ptr</span><span class=p>,</span><span class=w> </span><span class=n>V</span><span class=o>&&</span><span class=w> </span><span class=n>value</span><span class=p>);</span>
|
||||
</code></pre></div> <p>Only an <strong>editable</strong> document (<code class=highlight><span class=n>Editable</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=nb>true</span></code>, e.g. <a href=../../json_editable_document/ ><code>json_editable_document</code></a>) has <code>set</code>; calling it on a read-only <code>basic_json_document</code> fails to compile (<code class=highlight><span class=k>static_assert</span></code>).</p> <ol> <li>Replaces the value <code>target</code> refers to with <code>value</code>.</li> <li>Sets the member <code>key</code> of the object <code>object</code> to <code>value</code>: assigns it if <code>object</code> already has a member with this key -- the first one, should the key occur more than once, and the later duplicates are then dropped (see the <a href=#notes>Notes</a> below) -- or appends a new member at the end otherwise. A <a href=../../basic_json_view/is_null/ >null</a> <code>object</code> first becomes an empty object.</li> <li>Assigns <code>value</code> to the element at index <code>idx</code> of the array <code>array</code>, which must already exist (<code class=highlight><span class=n>idx</span><span class=w> </span><span class=o><</span><span class=w> </span><span class=n>array</span><span class=p>.</span><span class=n>size</span><span class=p>()</span></code>).</li> <li>Sets the value the <abbr title="JavaScript Object Notation">JSON</abbr> pointer <code>ptr</code> refers to, relative to <a href=../root/ ><code>root()</code></a>, to <code>value</code>. The <em>parent</em> of the target must already exist: an object member is set as in 2. (added if it does not exist yet), an array element is assigned as in 3., and a last reference token of <code class=highlight><span class=s>"-"</span></code>, or equal to the size of the array, appends <code>value</code> instead, exactly as <a href=../push_back/ ><code>push_back</code></a> would. A <a href=../../basic_json_view/is_null/ >null</a> parent becomes what <a href=../../basic_json/operator%5B%5D/ ><code>basic_json::operator[]</code></a> with a <abbr title="JavaScript Object Notation">JSON</abbr> pointer makes of it: an array if the last reference token is <code class=highlight><span class=s>"-"</span></code> or consists of digits only (for an index beyond 0, the array is first filled with null values up to that index), an object otherwise. An empty <code>ptr</code> sets <a href=../root/ ><code>root()</code></a> itself, as in 1.</li> </ol> <p>In every overload, <code>value</code> is accepted three ways: a <a href=../../basic_json_view/ ><code>basic_json_view</code></a> of <em>any</em> document -- read-only or editable, and it does not have to be <code>target</code>'s/<code>object</code>'s/<code>array</code>'s own document -- which is copied so that nothing is shared with the source document afterward; a <code>BasicJsonType</code> value; or anything <code>BasicJsonType</code> can be constructed from (numbers, strings, <code class=highlight><span class=kt>bool</span></code>, <code class=highlight><span class=nb>nullptr</span></code>, containers, ...).</p> <h2 id=template-parameters>Template parameters<a class=headerlink href=#template-parameters title="Permanent link">¶</a></h2> <dl> <dt><code>V</code></dt> <dd>the type of <code>value</code>, deduced; see above for what is accepted.</dd> <dt><code>I</code></dt> <dd>an integral type other than <code class=highlight><span class=kt>bool</span></code>, deduced (overloads taking a <code class=highlight><span class=kt>bool</span></code> or a non-integral type for <code>idx</code> do not participate in overload resolution).</dd> </dl> <h2 id=parameters>Parameters<a class=headerlink href=#parameters title="Permanent link">¶</a></h2> <dl> <dt><code>target</code> (in)</dt> <dd>the value to replace</dd> <dt><code>object</code> (in)</dt> <dd>the object (or null value) whose member to set</dd> <dt><code>array</code> (in)</dt> <dd>the array whose element to assign</dd> <dt><code>key</code> (in)</dt> <dd>the key of the member to set</dd> <dt><code>idx</code> (in)</dt> <dd>the index of the element to assign; a negative value throws (see <a href=#exceptions>Exceptions</a>)</dd> <dt><code>ptr</code> (in)</dt> <dd>a <abbr title="JavaScript Object Notation">JSON</abbr> pointer to the value to set, relative to <code>root()</code></dd> <dt><code>value</code> (in)</dt> <dd>the new value</dd> </dl> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">¶</a></h2> <ol> <li>a view of <code>target</code>, now holding <code>value</code></li> <li>a view of the member <code>key</code> of <code>object</code>, now holding <code>value</code></li> <li>a view of the element <code>idx</code> of <code>array</code>, now holding <code>value</code></li> <li>a view of the value <code>ptr</code> refers to, now holding <code>value</code></li> </ol> <h2 id=exception-safety>Exception safety<a class=headerlink href=#excepLine truncated
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_view.hpp></span>
|
||||
|
||||
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
|
||||
<span class=k>using</span><span class=w> </span><span class=n>json_editable_document</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_editable_document</span><span class=p>;</span>
|
||||
<span class=k>using</span><span class=w> </span><span class=n>json_editable_view</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_editable_view</span><span class=p>;</span>
|
||||
|
||||
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=c1>// a configuration file, as it might be read from disk -- "price" is</span>
|
||||
<span class=w> </span><span class=c1>// written with a trailing zero that has no effect on its value</span>
|
||||
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>text</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
<span class=s> "name": "cache",</span>
|
||||
<span class=s> "host": "db1",</span>
|
||||
<span class=s> "port": 6379,</span>
|
||||
<span class=s> "price": 19.90,</span>
|
||||
<span class=s> "replicas": ["db2", "db3"],</span>
|
||||
<span class=s> "timeout": 30</span>
|
||||
<span class=s>}</span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=n>json_editable_document</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json_editable_document</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>text</span><span class=p>);</span>
|
||||
|
||||
<span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>set</span><span class=p>(</span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>()[</span><span class=s>"port"</span><span class=p>],</span><span class=w> </span><span class=mi>6380</span><span class=p>);</span><span class=w> </span><span class=c1>// (1) replace a value</span>
|
||||
<span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>set</span><span class=p>(</span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>(),</span><span class=w> </span><span class=s>"region"</span><span class=p>,</span><span class=w> </span><span class=s>"us-east"</span><span class=p>);</span><span class=w> </span><span class=c1>// (2) add a member</span>
|
||||
<span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>set</span><span class=p>(</span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>()[</span><span class=s>"replicas"</span><span class=p>],</span><span class=w> </span><span class=mi>0</span><span class=p>,</span><span class=w> </span><span class=s>"db4"</span><span class=p>);</span><span class=w> </span><span class=c1>// (3) assign an element</span>
|
||||
<span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>set</span><span class=p>(</span><span class=n>json</span><span class=o>::</span><span class=n>json_pointer</span><span class=p>(</span><span class=s>"/timeout"</span><span class=p>),</span><span class=w> </span><span class=mi>45</span><span class=p>);</span><span class=w> </span><span class=c1>// (4) via a JSON pointer</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// members stay in document order (the new one at the end), and a number</span>
|
||||
<span class=w> </span><span class=c1>// that was not itself edited keeps its exact spelling</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>().</span><span class=n>dump</span><span class=p>(</span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json_editable_view</span><span class=o>::</span><span class=n>number_format</span><span class=o>::</span><span class=n>source</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// the same edits on a plain json value: object_t is a std::map, so</span>
|
||||
<span class=w> </span><span class=c1>// parsing already sorted the keys, and dump() rewrites every number to</span>
|
||||
<span class=w> </span><span class=c1>// its shortest form, even "price", which was never touched</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>plain</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>text</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>plain</span><span class=p>[</span><span class=s>"port"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>6380</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>plain</span><span class=p>[</span><span class=s>"region"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"us-east"</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>plain</span><span class=p>[</span><span class=s>"replicas"</span><span class=p>][</span><span class=mi>0</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"db4"</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>plain</span><span class=p>[</span><span class=n>json</span><span class=o>::</span><span class=n>json_pointer</span><span class=p>(</span><span class=s>"/timeout"</span><span class=p>)]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>45</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>plain</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"cache"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"host"</span><span class=p>:</span><span class=w> </span><span class=s2>"db1"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"port"</span><span class=p>:</span><span class=w> </span><span class=mi>6380</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"price"</span><span class=p>:</span><span class=w> </span><span class=mf>19.90</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"replicas"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"db4"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"db3"</span>
|
||||
<span class=w> </span><span class=p>],</span>
|
||||
<span class=w> </span><span class=nt>"timeout"</span><span class=p>:</span><span class=w> </span><span class=mi>45</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"region"</span><span class=p>:</span><span class=w> </span><span class=s2>"us-east"</span>
|
||||
<span class=p>}</span>
|
||||
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"host"</span><span class=p>:</span><span class=w> </span><span class=s2>"db1"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"cache"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"port"</span><span class=p>:</span><span class=w> </span><span class=mi>6380</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"price"</span><span class=p>:</span><span class=w> </span><span class=mf>19.9</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"region"</span><span class=p>:</span><span class=w> </span><span class=s2>"us-east"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"replicas"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"db4"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"db3"</span>
|
||||
<span class=w> </span><span class=p>],</span>
|
||||
<span class=w> </span><span class=nt>"timeout"</span><span class=p>:</span><span class=w> </span><span class=mi>45</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../push_back/ >push_back</a> - append to an array</li> <li><a href=../insert/ >insert</a> - insert an element into an array</li> <li><a href=../erase/ >erase</a> - remove an object member, an array element, or the value a <abbr title="JavaScript Object Notation">JSON</abbr> pointer refers to</li> <li><a href=../root/ >root</a> - the view of the root value, the starting point of overload 4</li> <li><a href=../../basic_json_view/dump/ ><code>basic_json_view::dump</code></a> - serialize the document, keeping an untouched number's spelling with <code class=highlight><span class=n>number_format</span><span class=o>::</span><span class=n>source</span></code></li> <li><a href=../#edits>Edits</a> - what an edit guarantees, for every overload</li> <li><a href=../../../features/json_view/#editing-a-document>Editing a document</a> - why editable documents keep the source text's order and number spelling</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">¶</a></h2> <ul> <li>Added in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 21, 2026 08:21:29 UTC">October 21, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 Line truncated
|
||||
@@ -0,0 +1,181 @@
|
||||
# nlohmann::basic_json_document::set
|
||||
|
||||
```
|
||||
// (1)
|
||||
template<typename V>
|
||||
view_type set(view_type target, V&& value);
|
||||
|
||||
// (2)
|
||||
template<typename V>
|
||||
view_type set(view_type object, string_view_t key, V&& value);
|
||||
|
||||
// (3)
|
||||
template<typename I, typename V>
|
||||
view_type set(view_type array, I idx, V&& value);
|
||||
|
||||
// (4)
|
||||
template<typename V>
|
||||
view_type set(const json_pointer& ptr, V&& value);
|
||||
```
|
||||
|
||||
Only an **editable** document (`Editable == true`, e.g. [`json_editable_document`](https://json.nlohmann.me/api/json_editable_document/index.md)) has `set`; calling it on a read-only `basic_json_document` fails to compile (`static_assert`).
|
||||
|
||||
1. Replaces the value `target` refers to with `value`.
|
||||
1. Sets the member `key` of the object `object` to `value`: assigns it if `object` already has a member with this key -- the first one, should the key occur more than once, and the later duplicates are then dropped (see the [Notes](#notes) below) -- or appends a new member at the end otherwise. A [null](https://json.nlohmann.me/api/basic_json_view/is_null/index.md) `object` first becomes an empty object.
|
||||
1. Assigns `value` to the element at index `idx` of the array `array`, which must already exist (`idx < array.size()`).
|
||||
1. Sets the value the JSON pointer `ptr` refers to, relative to [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md), to `value`. The *parent* of the target must already exist: an object member is set as in 2. (added if it does not exist yet), an array element is assigned as in 3., and a last reference token of `"-"`, or equal to the size of the array, appends `value` instead, exactly as [`push_back`](https://json.nlohmann.me/api/basic_json_document/push_back/index.md) would. A [null](https://json.nlohmann.me/api/basic_json_view/is_null/index.md) parent becomes what [`basic_json::operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) with a JSON pointer makes of it: an array if the last reference token is `"-"` or consists of digits only (for an index beyond 0, the array is first filled with null values up to that index), an object otherwise. An empty `ptr` sets [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) itself, as in 1.
|
||||
|
||||
In every overload, `value` is accepted three ways: a [`basic_json_view`](https://json.nlohmann.me/api/basic_json_view/index.md) of *any* document -- read-only or editable, and it does not have to be `target`'s/`object`'s/`array`'s own document -- which is copied so that nothing is shared with the source document afterward; a `BasicJsonType` value; or anything `BasicJsonType` can be constructed from (numbers, strings, `bool`, `nullptr`, containers, ...).
|
||||
|
||||
## Template parameters
|
||||
|
||||
`V` : the type of `value`, deduced; see above for what is accepted.
|
||||
|
||||
`I` : an integral type other than `bool`, deduced (overloads taking a `bool` or a non-integral type for `idx` do not participate in overload resolution).
|
||||
|
||||
## Parameters
|
||||
|
||||
`target` (in) : the value to replace
|
||||
|
||||
`object` (in) : the object (or null value) whose member to set
|
||||
|
||||
`array` (in) : the array whose element to assign
|
||||
|
||||
`key` (in) : the key of the member to set
|
||||
|
||||
`idx` (in) : the index of the element to assign; a negative value throws (see [Exceptions](#exceptions))
|
||||
|
||||
`ptr` (in) : a JSON pointer to the value to set, relative to `root()`
|
||||
|
||||
`value` (in) : the new value
|
||||
|
||||
## Return value
|
||||
|
||||
1. a view of `target`, now holding `value`
|
||||
1. a view of the member `key` of `object`, now holding `value`
|
||||
1. a view of the element `idx` of `array`, now holding `value`
|
||||
1. a view of the value `ptr` refers to, now holding `value`
|
||||
|
||||
## Exception safety
|
||||
|
||||
Basic exception safety: `value` is fully encoded -- including the checks below -- into storage owned by the document before anything already reachable from [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) is touched, so a failure while encoding `value` (an invalid argument, or `std::bad_alloc`) leaves the document completely unchanged, other than memory allocated for the encoding that is not reclaimed. A failure of a later allocation -- while an edited array or object switches from its parsed layout to a growable block, see [Notes](#notes) -- can still leave a partial effect, such as a [null](https://json.nlohmann.me/api/basic_json_view/is_null/index.md) `object`/`array` argument already turned into an empty object/array even though `value` itself was not linked in. Likewise, a failure of `value` in 4. leaves a null parent that is set with an index beyond 0 already filled with the null values before the index.
|
||||
|
||||
## Exceptions
|
||||
|
||||
1. Throws [`type_error.302`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error302) if `value` is a [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) view, or a [discarded](https://json.nlohmann.me/api/basic_json/is_discarded/index.md) `BasicJsonType` value (e.g. `BasicJsonType(value_t::discarded)`) -- an object or array `value`, of either kind, is fine and is encoded as a whole subtree.
|
||||
1. Throws [`type_error.305`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error305) if `object` is neither an object nor null -- the same message [`operator[]`](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/index.md) throws for a string argument on such a value. Throws [`type_error.316`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error316) if `key` is not valid UTF-8, with the same message [`BasicJsonType::dump()`](https://json.nlohmann.me/api/basic_json/dump/index.md) gives for that string. Also throws what 1. throws for `value`.
|
||||
1. Throws `type_error.305` if `array` is not an array -- the same message `operator[]` throws for a numeric argument on such a value. Throws [`out_of_range.401`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range401) if `idx` is negative, or if `idx >= array.size()`. Also throws what 1. throws for `value`.
|
||||
1. Throws what [`at`](https://json.nlohmann.me/api/basic_json_view/at/index.md) throws (overload 3) for resolving `ptr`'s parent, except that a missing object member or an array index equal to the array's size at the very last reference token is not an error there (it becomes a new member or an appended element) instead of [`out_of_range.403`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range403)/[`out_of_range.402`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range402). For the last reference token itself: if the parent is an object (or a primitive value, where it throws `type_error.305`), throws what 2. throws; if the parent is an array, throws what 3. throws for an index that is out of range, or, for a token that is not a valid array index, [`parse_error.106`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error106) (a leading `'0'`), [`parse_error.109`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error109) (not a number), [`out_of_range.410`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range410) (too large for `size_type`), or [`out_of_range.404`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range404) (an empty token); the same errors are thrown for a null parent and a token of digits (the parent is not changed then), and [`out_of_range.401`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range401) if the index is 4294967295 or more. Also throws what 1. throws for `value`.
|
||||
|
||||
Every overload also throws [`type_error.319`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error319) if `value` is (or contains) a binary value -- `BasicJsonType` can hold one, but a `json_document` cannot -- and [`invalid_iterator.202`](https://json.nlohmann.me/home/exceptions/#jsonexceptioninvalid_iterator202) ("view does not belong to this document") if `target`/`object`/`array` is a [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) view or a view of a *different* document (overloads 1-3 only; overload 4 always starts from this document's own [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md)).
|
||||
|
||||
## Complexity
|
||||
|
||||
1. Linear in the size of `value` (encoding it into the document's storage): constant for a scalar, linear in the number of nested values for an array or object. If `target` is itself an array or object that spans more than one node in its parent's original, unedited layout, and `value` is a scalar, replacing it additionally costs time linear in the size of the document, the *first* time (the parent of `target` is looked up from [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md), and then switches to links) -- see [Notes](#notes). Once the parent has links, `target` is replaced in constant time: setting every element of a large array one after the other is linear overall. To avoid the lookup altogether, use 3. (or 2. for an object), which know the parent.
|
||||
1. Linear in the number of members of `object`, to find an existing member with `key`, plus the complexity of 1. for `value`.
|
||||
1. Constant, plus the complexity of 1. for `value`.
|
||||
1. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at that level or the index into the array (as [`at`](https://json.nlohmann.me/api/basic_json_view/at/index.md)), plus the complexity of 2. or 3. for the last token; for a null parent and an index, linear in the index.
|
||||
|
||||
## Notes
|
||||
|
||||
Duplicate keys
|
||||
|
||||
If `object` already has more than one member with `key` (2.), the *first* one is assigned `value` and every later member with the same key is removed -- so that a lookup, an iteration, and [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) of `object` afterward all agree on a single value for `key`, the same way [`operator[]`](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/index.md) already picks the first occurrence of a duplicate key for reading. See the [Notes on duplicate keys](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/#notes) of `operator[]`.
|
||||
|
||||
Setting a member (2.) or an element (3., through 4.) of an array or object whose elements have not been edited before switches it from its parsed layout to a growable block holding links to its elements; a later [`push_back`](https://json.nlohmann.me/api/basic_json_document/push_back/index.md) or `set` on the same container reuses that block, growing it (amortized constant time) only once it runs out of room. This never moves an element itself -- only where the container's *links* to its elements live -- so a view of an element stays valid, but any iterator already taken over the container is invalidated, since it was walking the old layout. See [Edits](https://json.nlohmann.me/api/basic_json_document/#edits) for what stays valid across an edit in general.
|
||||
|
||||
The same switch happens, for the same reason, when overload 1. replaces a multi-node array/object value with a scalar: the *parent's* element sequence is what has to switch to links, not `target` itself, because the parent originally stepped over `target`'s whole subtree by its node count, which no longer applies once `target` is a one-node scalar.
|
||||
|
||||
## Examples
|
||||
|
||||
Example: (1)/(2)/(3)/(4) replace a value, set a member, assign an element, set via a JSON pointer
|
||||
|
||||
The example below edits a small configuration document -- replacing a value, adding an object member, assigning an array element, and reaching a field through a JSON pointer -- and shows what [`dump()`](https://json.nlohmann.me/api/basic_json_view/dump/index.md) preserves that is lost once the same edits are made on a `BasicJsonType` value instead: the order object members were written in, and the exact spelling of a number that was never touched.
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json_view.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using json_editable_document = nlohmann::json_editable_document;
|
||||
using json_editable_view = nlohmann::json_editable_view;
|
||||
|
||||
int main()
|
||||
{
|
||||
// a configuration file, as it might be read from disk -- "price" is
|
||||
// written with a trailing zero that has no effect on its value
|
||||
const std::string text = R"({
|
||||
"name": "cache",
|
||||
"host": "db1",
|
||||
"port": 6379,
|
||||
"price": 19.90,
|
||||
"replicas": ["db2", "db3"],
|
||||
"timeout": 30
|
||||
})";
|
||||
|
||||
json_editable_document doc = json_editable_document::parse(text);
|
||||
|
||||
doc.set(doc.root()["port"], 6380); // (1) replace a value
|
||||
doc.set(doc.root(), "region", "us-east"); // (2) add a member
|
||||
doc.set(doc.root()["replicas"], 0, "db4"); // (3) assign an element
|
||||
doc.set(json::json_pointer("/timeout"), 45); // (4) via a JSON pointer
|
||||
|
||||
// members stay in document order (the new one at the end), and a number
|
||||
// that was not itself edited keeps its exact spelling
|
||||
std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << "\n\n";
|
||||
|
||||
// the same edits on a plain json value: object_t is a std::map, so
|
||||
// parsing already sorted the keys, and dump() rewrites every number to
|
||||
// its shortest form, even "price", which was never touched
|
||||
json plain = json::parse(text);
|
||||
plain["port"] = 6380;
|
||||
plain["region"] = "us-east";
|
||||
plain["replicas"][0] = "db4";
|
||||
plain[json::json_pointer("/timeout")] = 45;
|
||||
std::cout << plain.dump(2) << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"name": "cache",
|
||||
"host": "db1",
|
||||
"port": 6380,
|
||||
"price": 19.90,
|
||||
"replicas": [
|
||||
"db4",
|
||||
"db3"
|
||||
],
|
||||
"timeout": 45,
|
||||
"region": "us-east"
|
||||
}
|
||||
|
||||
{
|
||||
"host": "db1",
|
||||
"name": "cache",
|
||||
"port": 6380,
|
||||
"price": 19.9,
|
||||
"region": "us-east",
|
||||
"replicas": [
|
||||
"db4",
|
||||
"db3"
|
||||
],
|
||||
"timeout": 45
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [push_back](https://json.nlohmann.me/api/basic_json_document/push_back/index.md) - append to an array
|
||||
- [insert](https://json.nlohmann.me/api/basic_json_document/insert/index.md) - insert an element into an array
|
||||
- [erase](https://json.nlohmann.me/api/basic_json_document/erase/index.md) - remove an object member, an array element, or the value a JSON pointer refers to
|
||||
- [root](https://json.nlohmann.me/api/basic_json_document/root/index.md) - the view of the root value, the starting point of overload 4
|
||||
- [`basic_json_view::dump`](https://json.nlohmann.me/api/basic_json_view/dump/index.md) - serialize the document, keeping an untouched number's spelling with `number_format::source`
|
||||
- [Edits](https://json.nlohmann.me/api/basic_json_document/#edits) - what an edit guarantees, for every overload
|
||||
- [Editing a document](https://json.nlohmann.me/features/json_view/#editing-a-document) - why editable documents keep the source text's order and number spelling
|
||||
|
||||
## Version history
|
||||
|
||||
- Added in version 3.13.0 unreleased.
|
||||
Reference in new issue
Block a user